Master / Detail with Entity Framework

Question

I have a table, say STUDENTS, that is related one-to-many to another table, CLASSES. I created an ASP.Net form to take input of a new student, including selection of one of the classes from a combo box. This is a small but multi-tier app, and I have started using the entity framework. So the EF generated classes for both those tables. On Submit, I am building a STUDENT object and handing it off to my Data controller for inserts, updates, etc.

In the STUDENTS table I have a CLASSID that's a FK to the CLASSES table. Back in the day, when I would do the insert to STUDENTS I would take the SelectedValue from the combo box and stick that into the ClassId field. However, with the EF, the STUDENT object doesn't have this ID field. Instead it has a CLASS field, which is a full-on instance of that class. When the new STUDENT object is built, that CLASS field is null. I want to populate it with the class the user chose in the combo.

I can't leave it null, and I can't just put the ID in there. I tried doing a retrieve of that class record and using that, but I get an error that I can't use the same object with two different contexts. What am I really supposed to be doing here?

Answer 1

You can set EntityKey property of ClassReference:

Student.ClassReference.EntityKey = new EntityKey("myModel.ClassSet", "ID", classId);

EDIT

This is how to add student (I can't check it now, but it should work):

newStudent.BClassReference.EntityKey = new EntityKey("FormsEntities.BClasses", "BClassId", 13);

FormsEntities fe = new FormsEntities();
fe.AddToStudents(newStudent);
fe.SaveChanges();

Answer 2

I had to do some research on the EntityKey property, and then tried to get this to work but I'm getting various kinds of errors. Basically, I create a new Student object and fill in all the values. Then I do this (I'm using a constant here for testing purposes):

BClass cls = new BClass();
cls.EntityKey = new EntityKey("FormsEntities.BClasses", "BClassId", 13);
newStudent.BClass = cls;
cls.BClassId = 13;

FormsEntities fe = new FormsEntities();
fe.Attach(newStudent.BClass);
fe.AddToStudents(newStudent);
fe.SaveChanges();

Earlier, I just tried to SaveChanges() but then I got an error saying there was already a key so I needed to use Attach(). So I added the Attach() line above and now I'm getting an error that says "An object with the same key already exists in the ObjectStateManager. The existing object is in the Unchanged state. An object can only be added to the ObjectStateManager again if it is in the added state."

Honestly, why is something so simple, so difficult? I've wasted many hours on this, when all I used to have to do is put the integer right in there.

Answer 3

How about start by doing things the easy way?

using (var fe = new FormsEntities())
{
    newStudent.BClass = fe.BClasses.Where(bc => bc.BClassId == 13).First();
    fe.AddToStudents(newStudent);
    fe.SaveChanges();
}

Don't muck around with EntityKeys until you get the basics working. I think that "optimizing" a save is largely a waste of time.